This invention relates to stainless steel alloys having excellent mechanical and corrosion resistant properties in which the nickel content is lower than prior art alloys having substantially equal corrosion resistance.
For centuries the principal metals employed in the manufacture of small arms were brasses, bronzes, iron and pack-carburized carbon steel for springs and other hardened components. Products of combustion of the black gunpowder employed at that time as propellant included nitrates and sulfates, both of which in conjunction with the moisture from the air, caused rusting of the iron and steel if not removed within a matter of hours. The brasses and bronzes did not corrode rapidly, but were relatively scarce, expensive and unsuited for most gun parts due to their low static and impact strengths.
By the time of the American Civil War, the flinlock method of ignition had been almost entirely supplanted by the percussion system in which the percussion primer caps contained chlorates to ignite the gunpowder. During ignition, these chlorates formed chlorides, which were even more corrosive to iron and steel than were the combustion products of the black powder itself.
During most of this period bluing and browning methods were employed to provide very thin coatings of iron oxides on the surfaces of iron and steel parts. Such coatings were more cosmetic than protective and would quickly rust through into the base metal if the guns were not quickly cleaned after firing with hot, perhaps soapy, water.
The Spencer rifles and carbines of the Civil War had their receivers coated with a thin layer of tin to help prevent attack of the metal on the outside surfaces. This of course did nothing to protect the bare and inside surfaces and working parts of the guns.
In the early part of the twentieth century, stypnates and other compounds were discovered to replace the earlier substances as priming compounds. Black gunpowder had also been replaced by other propellant compounds, so that the sources of corrosion caused by the firing of the guns themselves had been practically eliminated. However, the older corrosive priming compounds are still employed in many countries of the world for various reasons and may often be used by NATO forces. Even during the black gunpowder era, nickel coating of steel had been attempted in order to protect gun surfaces somewhat from rusting. However, the nickel coatings were relatively soft, somewhat permeable and not suitable for coating the bores of the weapons, but they did impede rusting due to the chlorides that come from human skin during handling of the weapons. On the other hand, such coatings eventually allow corrosion, especially in salt air, will sometimes come off when cleaned by certain powder fouling solvents and abrade away, e.g., by holster wear.
Chromium coatings of steel chambers, bores and bolt facings have also been employed, but these present certain problems. They are difficult to apply in even coatings, they may flake off in time, and they do present some buildup of thickness on the steel surfaces so that machining allowances must be made. The chromium coating of entire surfaces of gun parts is complicated, time consuming and not entirely successful.
Phosphate conversion coatings, referred to as Parkarizing, were employed during World War II for the protection of outside surfaces of small arms. Such coatings are generally more resistant to corrosion than bluing or browning, but they cannot be employed for internal working parts and will soon fail in the presence of chlorides.
Organic coatings have also been employed for exterior surfaces of steel parts. These coating are tough and corrosion resistant for exterior surfaces but are unsuitable for protection of bores and most internal parts and are relatively thick.
In addition to their susceptability to corrosion, steels also fail in time in rifle and machine gun barrels, and to a much lesser extent in pistol barrels, due to the erosion caused by the firing of the weapons. The burning of modern propellants causes the formation of compounds of nitrogen and carbon at very high temperatures. In rifles and machine guns maximum chamber pressures and temperature are of the order of 50,000 psi and 5,000.degree. F. For example, under these intense temperatures and pressures, nitrogen from the propellant combustion products combines chemically with the interior bore metal to form extremely hard, brittle layers of iron nitride. These nitride surface layers, during repeated firings, are subjected to stresses beyond their endurance limits. As a result, these nitride layers eventually flake off and are replaced by renewed layers, with the gradual removal of base metal for a distance of one or more inches just forward of the cartridge chamber. Weapon accuracy eventually deteriorates after about 10,000 rounds in rifle barrels. The temperatures of steel barrels under sustained full-automatic fire climb rapidly and accelerate the nitride deterioration. Police and military service pistol barrel life will vary from about 12,000 to 20,000 rounds under the best conditions.
In the Vietnam era, the United States light weight all-purpose M-60 machine gun was produced with a chromium-plated barrel bore and a six-inch investment-cast insert at the rear of the barrel made of an alloy of nominally 60% Co--27% CR--5.5% Mo--3% Ni--3% Fe--0.25% C. While this alloy was reported to give exceptionally long barrel life under sustained fire, it is not suitable for a variety of gun parts and is very expensive and difficult to fabricate.
The twelve to fourteen percent chromium stainless steels have been employed in rifles, pistols and shot guns. It is a well established fact that large quantities of nickel, cobalt, and chromium and smaller amounts of molybdenum, columbium (niobium) and silicon, tend to retard the carburizing and nitriding processes. Even these low chromium levels are slightly efficacious in improving rifle and machine gun barrel life. However, the main reason for employing these stainless steels has been their beneficial effect upon corrosion retardation. While the 12% Cr-type stainless steels are a significant improvement in this capacity, they fail where most needed. Specifically, they are depassivated by chlorides and are subject to pitting and crevice corrosion attack if allowed to remain in the presence of chloride for even short periods. Hence, these steels are not as well suited as hoped for in service that encounters salt water or even salt air such as for some police units, navy or other military units operating in or transported through salt or brackish water conditions. Even undercover officers and operatives who have cause to carry pistols in a manner that would encounter human perspiration would welcome weapons that were made of metals that truly resist chlorides. In all of these instances, the concentrations of chlorides may become quite high due to repeated wetting and drying, since these salts do not evaporate. Even in semiautomatic firing gun parts may become quite hot, so that chloride corrosion is accelerated by the heat.
Recently, the Glock 17 model pistol was developed and manufactured in Austria. The receiver and several of its parts are injection molded of high strength plastic, which is totally resistant to all common causes of corrosion including chlorides. There are several metallic parts in the mold at the time of injection, so that these become part of the final receiver. These parts as well as the slide, barrel and other components are all formed of steel, which is again susceptible to the various forms of corrosion.
Titanium, because it is totally immune to corrosive action by seawater or other chlorides, would appear to be a good choice for various weapons. It is, however, a very tough metal, quite difficult to fabricate and machine, and considerably less dense than steel. Titanium's low density results in a lighter weapon but one having heavier recoil. Even in the case of revolvers, which are of much simpler design than semiautomatic or automatic weapons, the sheet metal forming and welding practices employed in the manufacture of modern military weapons would be impractical to employ using titanium. Nevertheless, a revolver made almost entirely of titanium alloy has recently been introduced. The cylinder is machined from solid bar stock, while the frame and barrel are vacuum investment castings. Thus, while titanium may eventually be employed in some small arms parts, it is relatively too expensive and difficult to fabricate for general use at present.
Binder, U.S. Pat. No. 2,777,766, revealed alloys resistant to many corrosive materials, including dilute chloride solutions at or near room temperature. However, Binder's alloys do not resist hotter or more concentrated chlorides, possibly because they contain columbium, which is now known to lower chloride resistance. Henthorne, et al., U.S. Pat. No. 4,201,575, and DeBold, et al., U.S. Pat. No. 4,487,744, both disclose, in a sense, derivatives of the alloys of Binder. Henthorn's alloys were specifically developed to resist acid chlorides, but are not resistant under the conditions employed in the ASTM G-48 ferric chloride test at room temperature nor either the ASTM A 262-C boiling nitric acid test in the as cast condition or when sensitized at 1400.degree. F. for five minutes. In addition, the alloys developed by Henthorn are somewhat unbalanced and readily form sigma phase, which is quite detrimental to chloride corrosion resistance. Hence, those alloys are not well suited for many gun parts which are prepared by precision casting or forming and welding. Those alloys also have yield strengths that are too low for casting purposes.
DeBold in turn attempted to avoid the tendency of the alloys of Henthorn to form sigma phase. While the alloys of U.S. Pat. No. 4,487,744 possess resistance to a broad spectrum of corrosive agents, many of them also display poor resistance in the ASTM G-48 ferric chloride test at room temperature. Those alloys that best resist crevice corrosion at room temperature do not fare well in the pitting tests at 100.degree. F., and vice versa. Furthermore, the alloys of U.S. Pat. No. 4,487,744 must be solution annealed at about 1950.degree. F. and water quenched to develop their chloride resistance. Also, they have yield strengths which are too low for precision cast parts.
My own alloys of U.S. Pat. No. 4,765,957 may be formulated to have higher yield strengths than the alloys discussed above and resist seawater quite well, but they have a tendency to form sigma phase and do not resist chlorides at higher concentrations and higher temperatures.
My copending application, Ser. No. 176,409, filed Apr. 1, 1988, describes iron-based alloys of generally lower molybdenum and copper contents than the alloys of U.S. Pat. No. 4,765,957, in which nickel plus cobalt contents must exceed chromium contents by at least about 2% by weight. However, even these alloys generally have yield strengths that are too low for cast gun parts except when molybdenum contents are at a maximum, in which case there is a tendency for the austenite to destabilize and form sigma phase.
My copending application, Ser. No. 947,427 filed Dec. 29, 1986, describes an iron-base alloy of approximately 18% Cr, 7.5% Mo and certain other elements. While those alloys are superior in many ways to the alloys of Liljas, et al., U.S. Pat. No. 4,078,920, and Rossomme, et al., U.S. Pat. No. 4,421,557, all three alloys being of somewhat similar chemical compositional ranges, all three alloys still fail badly in the ASTM G48 ferric chloride test whether in the as cast condition or after welding without drastic post-weld solution annealing and quenching.
Baumel, U.S. Pat. No. 3,726,668, discloses welding rod filler alloys claiming very broad ranges of nickel, chromium and molybdenum, with the optional addition of copper. Baumel claims such weld materials provide excellent resistance to fluids which contain chloride ions. Baumel's actual examples present the welding of type 317 stainless steel and of type 317 stainless steel with a small addition of titanium, using filler rods made up of virtually type 317 stainless steel, except that the molybdenum contents are 4.3% and 4.2% instead of the 4% found in standard type 317. By today's standards, such alloys are considered to have very inferior resistance to seawater or similar chloride solutions. Baumel gives preferred compositional ranges of 15.0-20.0% chromium, 10.0-16.0% nickel and 3.5-5.0% molybdenum with what amounts to optional copper contents of 0.01-1.5%. While Baumel's exemplary alloys would also be essentially austenitic, most of the alloys represented by his claimed ranges of elements would contain large amounts of sigma or other additional undesired phases.
Yamaguchi, et al. U.S. Pat. No. 4,141,762, provides for two-phase alloys having a high manganese content, none of which have much resistance to any but the weakest chloride solutions. In the ferric chloride test of ASTM G-48, such two-phase alloys fail catastrophically in less than three days.
Goda, et al., U.S. Pat. No. 3,811,875, claims very broad contents of nickel and chromium but optional molybdenum contents only up to 3.5%. Abo, et al. U.S. Pat. No. 4,172,716, also claims broad ranges of many elements, including nickel and chromium but makes molybdenum and copper contents optional. Kudo, et al., U.S. Pat. No. 4,400,349, is somewhat similar in claiming broad ranges of nickel and chromium and in making molybdenum and copper optional additions.
Thus, there has remained a need for ductile, strong, weldable, readily fabricable alloys that are resistant to hot chloride solutions as well as ordinary corrosive substances as are found in air and water. The alloys of this invention are directed toward that end, although they also have excellent resistance to a variety of other substances.
More particularly, a need has remained in the art for alloys of relatively low cost and ease of fabrication that can be used in the manufacture of small arms employing corrosive cartridge primers and which are immune to corrosion in the presence of hot chlorides.